test: admin role separation, batch operations, allowance cleanup, error handling

[privexlabs]

Summary

Adds the four test suites requested for this wave: integration coverage for
admin role separation and batch-style operations on the ChainLearn
contracts, plus unit coverage for expired-allowance storage cleanup and
typed error handling.

Changes

• tests/integration/admin_role_flow.rs (new): verifies that the Minter
  and Pauser roles on learn-token are enforced independently (a minter
  can mint but not pause; a pauser can pause/unpause but not mint), that
  granting/revoking roles takes effect immediately, and that role_granted
  / role_revoked events are emitted with the correct address.
• tests/integration/batch_operations.rs (new): verifies
  batch_claim_reward processes multiple quizzes independently (an
  already-claimed quiz is skipped without blocking the others), and
  exercises batch-style module completion and quiz submission flows
  against progress-tracker.
• tests/unit/token_tests.rs: adds a test that cleanup_expired_allowances
  removes only expired allowances, preserves valid ones, shrinks the
  spender registry accordingly, and is a no-op on a second pass.
• tests/unit/token_tests.rs, tests/unit/progress_tests.rs,
  tests/unit/credential_tests.rs: each adds a test asserting that
  calling initialize a second time returns the typed
  ContractError::AlreadyInitialized error (via try_initialize) rather
  than an untyped panic.
• contracts/learn-token/src/lib.rs: re-exports AdminRole alongside the
  existing Proposal/VestingSchedule re-exports so external tests can
  reference it, since it's defined in the crate's private storage
  module.
• Cargo.toml: registers the two new integration test files as
  [[test]] targets, matching the existing pattern for every other test
  file in the repo.

Issues

Resolves #282
Resolves #283
Resolves #284
Resolves #285

Verification

Changes were reviewed manually: git status, git diff --stat, and the
full git diff were inspected line by line against the current contract
source (role/storage/event APIs) to confirm every call matches existing
function signatures and conventions. cargo build and cargo test were
NOT run, per instructions; no test execution or compilation was performed.

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